Semiconductor diodes exhibit a forward conduction threshold voltage effect that is characteristic, among other things, of the material of which the diode is made. The threshold voltage of a given diode is generally considered to correspond to the so-called "knee" of the diode forward conduction characteristic curve and is typically about 700 millivolts at room temperature for a small signal silicon diode or silicon transistor junction. The threshold voltage of germanium diodes or of Shottky barrier silicon diodes is, typically, about half that of a conventional PN junction silicon diode.
In certain applications the threshold voltage of a semiconductor diode is used to establish a reference level for processing of electrical signals. As an example, in the audio noise reduction system proposed by Christopher et al. in U.S. patent application Ser. No. 229,743 filed Jan. 29, 1981, an adaptive filter for an audio signal expander is disclosed wherein a semiconductor diode is used as a threshold switch for modifying the filter time constant under certain dynamic signal conditions. As another example, type IN914 silicon diodes are employed as threshold devices in the adaptive filter of an audio expander circuit described by J. Roberts in the article "$70 Decoder for New CX Records" published in the January, 1982, issue of POPULAR ELECTRONICS magazine, pp. 39-44.
In the interest of cost reduction and improved reliability, it would be desirable to implement syllabic audio expanders, such as those described above, in integrated circuit form. It is further desirable to operate the integrated circuit at a relatively low supply voltage level so as to obtain benefits such as reduced power consumption, reduced heat build-up, and improved reliability. Generally speaking, circuits which are designed to operate at a given supply voltage when using conventional silicon transistors or diodes as threshold conduction devices may be "scaled down", so to speak, to operate at a lesser supply voltage by substituting diodes having a proportionally lower threshold voltage (e.g., germanium diodes, Shottky barrier silicon diodes, etc.) for the silicon diodes. Such a substitution, however, presents certain practical difficulties with regard to integrated circuit fabrication.
As an example, direct substitution of diodes of different materials or construction on the integrated circuit "chip" may require a further processing step (e.g., an added metalization or a deposition of a further semiconductor material). This may reduce the process yield. An alternative of connecting the "substitute" diode to the integrated circuit via external pin connections avoids the need for extra processing steps but requires additional circuit pins which may not be available in the desired integrated circuit package. Also, additional assembly costs may be involved in connecting the substitute diode to the extra pins and reliability may be degraded by the added connections. A further difficulty is that the "substitute" diode may have forward or reverse bias conduction characteristics which are greatly different from the desired "scaled down" values.